Lightweight transport protocol

ABSTRACT

A smart NIC (Network Interface Card) is provided with features to enable the smart NIC to operate as an in-line NIC between a host&#39;s NIC and a network. The smart NIC provides pass-through transmission of network flows for the host. Packets sent to and from the host pass through the smart NIC. As a pass-through point, the smart NIC is able to accelerate the performance of the pass-through network flows by analyzing packets, inserting packets, dropping packets, inserting or recognizing congestion information, and so forth. In addition, the smart NIC provides a lightweight transport protocol (LTP) module that enables it to establish connections with other smart NICs. The LTP connections allow the smart NICs to exchange data without passing network traffic through their respective hosts.

RELATED APPLICATION

This is a divisional patent application of copending application Ser.No. 14/752,713, filed Jun. 26, 2015, entitled “LIGHTWEIGHT TRANSFERPROTOCOL”. The aforementioned application is hereby incorporated hereinby reference.

BACKGROUND

Data networking involves hardware and software. On the software side,networking protocols are often designed for current or near-termhardware capabilities. Protocols often become widely adapted while atthe same time networking hardware improves. Processors become moreefficient and capable and communication mediums gain reliability andcapacity. Over time, networking protocols designed in the past maybecome less suitable for the hardware that is available in the present.For example, a transport protocol might have mechanisms for ensuringreliability, responsiveness to congestion, and ordered delivery. Suchmechanisms are often not well suited for new types of networks. Yet, tomaintain continuous inter-network operability, older protocols are notpractical to modify. There is a need for techniques that can translateadvances in networking technology into improvements in the overallperformance of possibly older protocols without having to change thoseprotocols or their existing implementations.

Moreover, some capabilities of networking hardware have not been fullyappreciated and realized. So-called smart NICs (Network InterfaceCards), for example FPGA (Field Programmable Gate Array) NICs havebecome more common. These newer interfaces, like traditional NICs,provide physical and media connectivity. They also include additionalprocessing capability, sometimes in the form of reconfigurable circuitry(e.g., FPGAs). These processing-augmented NICs may allow features ofsome protocols to be offloaded from the host's CPU (Central ProcessingUnit) to the NIC. Some smart NICs may even allow an entire transportprotocol to be fully offloaded from a host's CPU to the smart NIC.However, this approach often requires significant host-side changes. Forexample, host-side software might need to be re-written to communicatedirectly with its NIC via a custom application programming interface(API) rather than via a standard transport protocol. In addition, mostsmart NICs with off-load functionality do not function as “bump in theline” (“in-line”) devices (i.e., devices that connect to a host'sexisting NIC). Thus, to upgrade a host from an ordinary NIC to a smartNIC, the host's ordinary NIC must be physically replaced with a newsmart NIC. Large scale upgrades can be expensive and disruptive. Thereis a need for smart NICs that can be provided to hosts to improvenetwork and host performance without necessarily requiring host-sidesoftware or hardware changes.

Moreover, as realized only by the inventors, the development of smartNICs has also made it possible to off-load application-levelfunctionality to smart NICs. For example, a distributed applicationmight have elements throughout a data center that need to exchange data.Some of those elements or their helper agents might be capable of beingexecuted by a smart NIC. However, it has not been realized thatapplication-level code (or other code) on in-line smart NICs might becapable of direct network communication without having to traverse theirrespective hosts and host network stacks.

Techniques related to the issues mentioned above are described below.

SUMMARY

The following summary is included only to introduce some conceptsdiscussed in the Detailed Description below. This summary is notcomprehensive and is not intended to delineate the scope of the claimedsubject matter, which is set forth by the claims presented at the end.

A smart NIC (Network Interface Card) is provided with features to enablethe smart NIC to operate as an in-line NIC between a host's NIC and anetwork. The smart NIC provides pass-through transmission of networkflows for the host. Packets sent to and from the host pass through thesmart NIC. As a pass-through point, the smart NIC is able to acceleratethe performance of the pass-through network flows by analyzing packets,inserting packets, dropping packets, inserting or recognizing congestioninformation, and so forth. In addition, the smart NIC provides alightweight transport protocol (LTP) module that enables it tocommunicate directly with other smart NICs without passing networktraffic through their respective hosts.

Many of the attendant features will be explained below with reference tothe following detailed description considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present description will be better understood from the followingdetailed description read in light of the accompanying drawings, whereinlike reference numerals are used to designate like parts in theaccompanying description.

FIG. 1 shows hosts arranged to communicate with each other via a datanetwork.

FIG. 2 shows high level features of an in-line smart NIC.

FIG. 3 shows another view of a smart NIC.

FIG. 4 shows details of a switch.

FIG. 5 shows details of a lightweight transport protocol (LTP) module.

FIG. 6 shows a process for transmitting application LTP data.

DETAILED DESCRIPTION

Embodiments described below relate to in-line smart NICs acceleratingnetwork flows of hosts for which they provide network connectivity.Embodiments described below also relate to enabling direct inter-NICnetwork communication. Technical details that support and enhance thesefeatures are also described below.

FIG. 1 shows hosts 100 arranged to communicate with each other via adata network 102. The hosts 100 may be any type of computing device thatcan operate as network flow endpoints. The form of a host 100 is notimportant (e.g., a blade, server, workstation, laptop, etc.), butprocessing hardware (e.g., a CPU, graphics processing units, etc.),storage hardware (e.g., memory, disk drives, etc.), and hardware fortheir cooperation (buses, input/output controllers and ports, etc.) areassumed. For purposes herein, details of the data network 102 are notsignificant. For example, the data network 102, at the physical/linklevel, might include segments of shared medium (e.g., Ethernet), one ormore switched fabrics (e.g. Infiniband or switched-fabric fibrechannel), token rings, etc. Any type of known data network 102 capableof providing network-level routing between the hosts 100 or otherdevices may be used. For discussion, the data network 102 will beassumed to be an Internet Protocol (IP) network that provides IP routingfor the hosts 100, which have respective IP addresses assigned to them.

The hosts 100 are provided with respective smart NICs 104. The hosts 100may have their own NICs (not shown), and the smart NICs 104 (which mayalso be referred to as “in-line NICs”) are configured to intermediatenetwork flows 106 between the host NICs and the data network 102. Thenetwork flows 106 can be Transmission Control Protocol (TCP)) flows, forexample. The smart NICs 104 are able to exchange network packets withthe data network 102 via media/physical links 105 and are able toexchange network packets with their respective hosts 100 viamedia/physical links (not shown) to the host NICs. As shown in FIG. 1,hosts 100 are able to communicate with each other (and other nodes)using the intermediated network flows 106. In short, a smart NIC 104 ofa host 100 will intermediate packets of network flows 106 that terminateat the host 100. As described further below, the type of intermediationprovided by a smart NIC 104 can range from basic pass-through logic thatdoes not attempt to affect its network flows 106 to pass-through logiccapable of improving the performance of its network flows 106.

As will also be described further below, the smart NICs 104 mayimplement a lightweight transport protocol (LTP). The LTP is aconnection-oriented protocol that provides reliability and ordering withlow overhead. As shown in FIG. 1, LTP connections 108 begin and end atthe smart NICs 104 and thus provide a form of inter-NIC communicationthat does not burden the hosts 100, since the hosts 100 do not receiveor otherwise handle LTP packets. The smart NICs 104 are able to, attheir own initiative, (i) open and close LTP connections 108, (ii)generate LTP packets, and (iii) send and receive LTP packets via the LTPconnections 108.

FIG. 2 shows high level features of an in-line smart NIC 104. Asmentioned above, a smart NIC 104 may be arranged in-line between a host100 and the data network 102 (as used herein “in-line” is a term used toidentify a type of smart NIC and does not imply that such a NIC ispresently connected to a host and network). In particular a smart NIC104 may have a first physical/link connection 120 physically connectingthe smart NIC 104 with its host 100. The smart NIC also has a secondphysical/link connection 122 connecting to the data network 102. Thephysical/link connections may each be any type, for instance Ethernet,Fibre Channel, InfinBand, etc. A physical/link connection may also be awireless medium. As discussed with reference to FIG. 3, the smart NIC isprovided with media access controllers (MACs) to interface with thephysical/link connections 120, 122.

To facilitate pass-through intermediation of network flows 106, thesmart NICs 104 are provided with one or more pass-through components124. A pass-through component 124 provides store-and-forward bufferingof flow packets. A pass-through component 124 may also include logic toimprove the performance of the flows that it is handling. As describedfurther below, a pass-through component 124 can use a variety of factorsto improve a flow's performance, including, for example, explicitcongestion notifications (ECNs), classifications of flows (e.g. lossyand lossless), information about the state of the network 102, featuresof the flows themselves, etc. As also described below, a variety ofactions may be taken to affect the behavior of flows, such as insertingpackets into flows, inserting priority flow control (PFC) markers intoflow packets, pausing or throttling flows, random early dropping (RED)of flow packets, etc.

A network flow 106 used by applications to exchange data may passthrough a smart NIC as follows. A host-based application 126 (anyapplication-layer code on a host 100) has data to convey. The data ispassed through an operating system API/facility (e.g., a stream orsocket) to a network stack 128, where the data is placed in transportpacket(s) (e.g., TCP packets), which are encapsulated in networkpacket(s) (e.g., IP packets with the host's IP address as the sender),which are in turn placed in the payload(s) of physical layer frames(e.g., Ethernet frames). The frames are passed through the firstphysical/link connection 120 to the smart NIC 104. The smart NIC 104strips the network frames, stores the transport packets, and optionallychecks the IP and/or transport packets for features such as ECN markers,stream-classification flags, etc. When buffered packets are ready to besent, the smart NIC encapsulates them in an IP packet with the samesource and destination addresses as the IP packets received from thehost. The IP packets are then framed for the second link/mediaconnection 122 and transmitted thereon. In effect, the IP packetstransmitted by the smart NIC to the data network are generally the sameas those originally received from the host.

Flow packets received by a smart NIC from the data network 102 aregenerally forwarded to the host in the same fashion, and the host'snetwork stack 128 similarly provides the application data to thehost-based application 126. Notably, the in-line arrangement of thesmart NIC and the bi-directional pack passing of packets allows a smartNIC and its host to use the same IP address. This can allow the host'ssmart NIC to be added or removed transparently to the host and to therouting of the data network. As discussed further below, LTP packets canalso use the hosts' IP addresses as source and destination addresses forIP routing, even though the LTP packets are not sent or received by thehosts.

To facilitate inter-NIC LTP connections 108, smart NICs 104 may beprovided with respective LTP modules 130. The LTP modules 130 implementthe LTP. An LTP module 130 receives a request from a NIC-basedapplication 132 to send data. The NIC-based application 132 can be anytype of code. For instance, the NIC-based application 132 might be anapplication that supports the cooperation between smart NICs. TheNIC-based application might be part of a distributed application that,through other means, shares data with a host-based application. In oneembodiment, the smart NICs have a bus interface to connect with a hostbus (e.g., a Peripheral Component Interconnect Express bus). Software onthe host can send data through the bus to its smart NIC. An agent on thesmart NIC interacts with the local LTP module 130 by: requesting aconnection to a remote LTP module (the request specifying thedestination network address, and providing data to the LTP module 130,which in turn packetizes the data into LTP packets and managestransmission of the LTP packets, as per the LTP, to the remote LTPmodule. In one embodiment, the LTP connections 108 are one-way (notbi-directional). In another embodiment, the LTP packets used by the LTPmodules 130 are UDP (Universal Datagram Protocol) packets that areultimately routed in IP packets. As described below, a smart NIC 104 caninclude an FPGA 131 or other form of programmable logic device. The FPGA131 can be configured to implement the LTP module 130, the pass-throughcomponents 124, the NIC-based application 132, or other elements of asmart NIC.

FIG. 3 shows another view of a smart NIC 104. The smart NIC 104 includesa first MAC 150, a second MAC 152, a bypass module 154, a switch 156,the LTP module 130, and a NIC-based application 132. The bypass module154 can implement basic “dumb” pass-through handling of flow packets forthe smart NIC. The switch 156 can extend the pass-through handling toinclude flow-processing functions such as injecting packets into flows,removing packets from flows, altering flow behavior, etc.

The bypass circuit 154 allows the smart NIC 104 to function as a basicbridge under conditions when flow processing might not be available. Forinstance, when the smart NIC is in a certain state such as a bootingstate, a resetting state, an error state, etc. A bypass control 158 canbe signaled to cause the MACs 150, 152 to either communicate directlyvia their respective FIFO buffers, or indirectly by passing packetsthrough the switch 156 for in-line processing. The switch 156 can alsosignal the bypass control 158 (see dashed line from switch 156 to bypasscontrol 158).

More specifically, the smart NIC may include error checking facilitiesto determine if the bypass should be triggered. If an error condition isdetected then the bypass mode is entered. Detectable conditions mightbe: a packet is being transmitted that doesn't have data ready (datastuttering), two end-of-packet flags occur without a start-of-packetflag between them, or two start-of-packet flags occur without anend-of-packet flag between them.

In addition to the above fixed conditions, the smart NIC can trackpacket ingress and egress counts on both MACs and can provideconfigurable minimum packets-per-interval levels that can indicate errorconditions. The ingress/egress counters can be used to determine iftraffic in either direction through the smart NIC has ceased to flow, orif internally produced traffic has stopped.

FIG. 4 shows details of the switch 156 (solid lines represent datapaths, and dashed lines represent control signals). The switch 156provides features to allow the smart NIC 104 to add packets to outgoingnetwork-bound flows and to prevent LTP packets from being sent on to thehost system. If the data network 102 supports several lossless classesof traffic, the switch 156 can be configured to provide sufficientsupport to buffer and pause incoming lossless flows to allow it toinsert its own traffic into the network. To support that, the switch 156can be configured to distinguish lossless traffic classes (e.g., RemoteDirect Memory Access (RDMA)) from lossy (e.g., TCP/IP) classes of flows.A field in a packet header can be used to identify which traffic classthe packet belongs to.

The switch 156 has a first port 170 (host-side) to connect to the firstMAC 150, and a second port 172 (network-side) to connect to the secondMAC 152. A third local port 174 provides internal service to the smartNIC, for example, to the LTP module 130. This port can applybackpressure if needed. The switch 156 may generally operate as anetwork switch, with some limitations. Specifically, the switch 156 maybe configured to pass packets received on the local port 174 (e.g., LTPpackets) only to the second port 172 (not the first port 170).Similarly, the switch 156 may be designed to not deliver packets fromthe first port 170 to the local port 174.

Internally, the switch 156 has two packet buffers; one for the receiving(Rx) first port 170 and one for the receiving second port 172. Thepacket buffers are split into several regions. Each region correspondsto a packet traffic class. As packets arrive and are extracted fromtheir frames (e.g., Ethernet frames) they are classified by packetclassifiers into one of the available packet classes (lossy, lossless,etc.) and written into a corresponding packet buffer. If no buffer spaceis available for an inbound packet then the packet will be dropped. Oncea packet is stored and ready to transmit, an arbiter selects from amongthe available packets and transmits the packet (the packet may be framedby the switch 156 or another element of the smart NIC). A PFC insertionblock allows the switch 156 to insert PFC frames between flow packets atthe transmit half of either of the ports 170, 172.

The switch 156 can handle a lossless traffic class as follows. Allpackets arriving on the receiving half of the first port 170 and on thereceiving half of the second port 172 should eventually be transmittedon the corresponding transmit (Tx) halves of the ports. Packets arestore-and-forward routed. Priority flow control (PFC) can be implementedto avoid packet loss. For lossless traffic classes, the switch 156 maygenerate PFC messages and send them on the transmit parts of the firstand second ports 170, 172. In one embodiment, PFC messages are sent whena packet buffer fills up. When a buffer is full or about to be full, aPFC message is sent to the link partner requesting that traffic of thatclass be paused. PFC messages can also be received and acted on. If aPFC control frame is received for a lossless traffic class on thereceive part of either the first or second port 170, 172, the switch 156will suspend sending packets on the transmit part of the port thatreceived the control frame. Packets will buffer internally until thebuffers are full, at which point a PFC frame will be generated to thelink partner.

The switch 156 can handle a lossy traffic class as follows. Lossytraffic (everything not classified as lossless) is forwarded on abest-effort basis. The switch 156 is free to drop packets if congestionis encountered. Random Early Drops (REDs) can be implemented on thereceive part of the first port 170. REDs can help cap host generatedlossy traffic to a target bandwidth. Limiting host bandwidth ensuresthat when the smart NIC starts introducing its own traffic there willnot be a corresponding sudden drop in available bandwidth, which wouldinteract poorly with TCP/IP streams using the link. A control parametercan be provided to set the RED thresholds.

As can be seen from features of the switch 156, a smart NIC 104 is ableto act as a throughput for its host while improving the performance ofthe host's flows. Signs of congestion can be detected in packets orflows before the packets traverse a host and its network stack. A smartNIC can react to those signs more quickly than the host. For example, ifa congestion marker is detected in a packet on its way to the host, thesmart NIC can quickly stop or start the flow, increase or decreaseavailable bandwidth, throttle other flows/connections, etc., beforeeffects of congestion start to manifest at the host. Moreover,congestion markers or the like can be removed from a flow before theyreach the host, thus reducing the flow-handling load at the host. As aresult, the host can run an ordinary network stack (e.g., TCP/IP) andmany of the high-overhead features of that stack can remain dormant. Forexample, window size adjustments can be minimized or avoided, lesspacket buffering may be needed, handling of malformed or inconsistentpackets may not be necessary (the smart NIC can intercept and removesuch packets and possibly request retransmission), congestion signalingcan become unnecessary at the host, the need for packet fragmentation orMTU adjustments, etc. Flow transformations such as compression,encryption, and flow tables can also be transparently assumed.

Before explaining the LTP module 130, it will be helpful to understandthe function of the LTP and how it can be used. Smart NICs, in general,are ordinary NICs that are augmented with processing hardware that mightnot be needed for the minimal networking functions of a NIC. Generalpurpose processors accompanied with appropriate software in staticmemory can be one form of processing hardware used to augment a NIC.Reconfigurable digital circuits are another form of processing hardwarethat are convenient due to their flexibility and speed. Suchreconfigurable circuits, referred to herein as programmable logicdevices (PLDs), can be FPGAs or known equivalents. NIC-basedsupplemental processing can also be provided with ASICs(application-specific integrated circuits), ASSPs (application-specificstandard parts), SoCs (system on a chip), etc.

A smart NIC, in particular one with a PLD, can be configured to act as athin autonomous node on a network. In one embodiment, a smart NIC can beconfigured with an environment or shell within which arbitrary processesor execution units can be instantiated. In any case, NIC-basedapplication-level code (or hardware) can be advantageous due to closeproximity to the network and lack of host-based burdens such as acomplex network stack, interrupt handling, resource sharing, etc. Inshort, NIC-based applications may be able to communicate with latenciesand throughputs close to the maximum possible on a network. Moreover,they may be capable of assured performance due to their use of dedicatedhardware. As noted above, such NIC-based application code/circuitry canalso cooperate with host-based software at high data rates if the smartNIC has a bus connection with the host. As can be seen, a smart NIC canitself be an agent of sorts that generates and consumes network trafficfor its own purposes. To take advantage of a smart NIC's locales andprocessing capabilities, it can be advantageous to provide such smartNICs with their own mechanism or protocol for directly exchanging datavia the data network.

FIG. 5 shows details of the LTP module 130. The LTP module 130 mayinclude one or more data buffers 190 to buffer payload, a transmitbuffer 192, a connection table 194, connection management logic 196, anddata transmission logic 198. These elements can operate to provideefficient and reliable connection-based communication between smartNICs.

The connection management logic 196 provides an interface with which aNIC-based application 132 can open a connection. The application passesa destination network IP address to the connection management logic 196.The connection management logic 196 creates a new entry in theconnection table 194. The entry includes an index or handle, thedestination IP address, and the current sequence number for theconnection (initially zero. An LTP connection request is then send tothe destination IP. The recipient smart NIC responds by adding acorresponding entry in its connection table. In one embodiment, eachsmart NIC also maintains a receiving connection table for connectionsopened by other smart NICs. While not necessary, use of both send andreceive connection tables can reduce overhead and possibly avoidexplicit identification of one-way vs two-way communication betweendevices.

In one embodiment, the smart NIC may include facilities for providingand managing streams or virtual channels for applications. In thisembodiment, the application's connection request may also specify orassociate a stream 200 with the requested connection. Data from thestream is then automatically passed to the connection as it becomesavailable. In another embodiment, the application itself passes data toits connection using the index/handle to specify the connection. Aconnection entry can be removed from the connection table 194 either byan explicit call from the application or by a monitor that removesconnections that have no activity for a defined time period.

FIG. 6 shows a process for transmitting application data 202 performedby the data transmitting logic 198. As noted above, application data 202is passed to the LTP module 130 either directly from the application orfrom a stream 200 associated with the connection. In the former case,the application passes the application data 202 and the handle of theconnection (e.g., index “5” in FIG. 5). At step 220, the applicationdata is received, and at step 222 the incoming data is added to acorresponding buffer in the data buffer 190. At step 224 the buffer ischecked. In one embodiment, the buffer is considered full when a nextportion of data (or the current application data 202) would fill thebuffer up to a size such as the maximum transmission unit (MTU) of thedata network. In short, incoming application data that is to betransmitted it buffered until there is sufficient data to form an LTPpacket. If step 224 determines that the buffer is not ready to transmit,then the data transmission logic 198 waits for additional applicationdata. To allow small messages to be sent with lower latency, the LTPmodule 130 may also be configured to receive a flag or signal thatindicates that the application data (e.g., a message) is complete, whichwill trigger transmission of an LTP packet.

Assuming that there is a sufficient amount of buffered data, at step 226an LTP payload is formed from the buffered data and the buffer is reset(the incoming data is either included in the payload or in the emptybuffer). At step 228 an LTP packet 204 is formed for the payload. Thesequence number for the relevant connection is incremented in theconnection table 194 and is added to the new LTP packet's header. Thedestination IP address is obtained from the appropriate entry in theconnection table 194 and is added to the LTP packet's header. In oneembodiment, the LTP packet 204 is formatted as a standard UDP packet.The UDP packet format is only a convenience, and the UDP standard itselfneed not be implemented. Because LTP packets are routed in IP packetsand are parsed only by the smart NICs, any format may be used for theLTP. If the UDP packet format is used, a port number can be defined toidentify LTP packets. In this way, the smart NIC can differentiatebetween LTP packets and UDP packets that are not LTP packets andtherefore are to be passed through. LTP control/overhead packets suchacknowledgment packets, connection setup and takedown packets, etc., arealso formatted to be identifiable as LTP packets (e.g., UDP packets witha specific port number). Note that in one embodiment, the LTP modulealso forms the IP packets that will carry the LTP packets. That is, theswitch receives a complete, well-framed, and formatted packet.

At step 230 the LTP packet 204 is copied to the transmit buffer 192 andthen forwarded to the local port 174 of the switch 156, which frames andtransmits the LTP packet 204 from the transmit part of its second port172 (the network-facing port). The LTP module provides ordered reliabledelivery. In one embodiment, acknowledgment (ACK) LTP packets 206 aretransmitted to confirm receipt. Each LTP ACK includes the sequencenumber of the LTP packet being acknowledged. When an LTP packet isacknowledged, it can be removed from the transmit buffer 192. In oneembodiment, negative acknowledgements (NAKS) and/or timeouts can be usedto trigger retransmission of buffered LTP packets.

The LTP module 130 also handles incoming LTP packets. When packetsarrive on the local port 174 from the switch 130, the LTP module 130determines which entry in the connection table 194 pairs with thepacket. If no matching entry exists the packet is dropped. The LTPmodule 130 checks that the incoming sequence number matches the expectedvalue from the connection table 194 entry. If the sequence number isgreater than the connection table entry, the packet is dropped. If thesequence number is less than expected, an LTP ACK is generated and thepacket is dropped. If the incoming LTP packet's sequence number matchesthe entry in the connection table 194 then the packet's payload isunloaded and passed to an application or a stream associated with theconnection (as the case may be). If the LTP module 130 is unable to passoff the payload data then the incoming LTL packet can be dropped. Oncethe incoming LTP packet has been accepted, an LTP ACK is generated andthe sequence number for the connection is incremented in the connectiontable 194. In another embodiment, inbound LTP packets can be buffered tore-order out-of-order packets. Also a NACK may be generated when adropped packet is detected (via a skipped sequence number, etc.).

Because the LTP traffic is handled by the switch 156, the LTP packetscan be classified and managed in the same way that the packets ofhost-based network flows can be managed. LTP connections can thereforeadhere to PFC stalls and ECNs, for instance. LTP traffic can also beprovided with a bandwidth amount or a quality of service setting. Unusedheader space, if available, can be used to send ECN and/or QCN/DC(Quantized Congestion Notification with Delay-Based CongestionDetection) information.

1. A first network interface card (NIC) to handle network flows for ahost, the first NIC comprising: a media access controller (MAC)configured to connect to a network; processing hardware configured totransmit packets of the network flows by transmitting to the network,from the MAC, packets received from the host, and by passing to the hostpackets received from the network by the MAC; a first module configuredto implement a lightweight transport protocol (LTP); and a second moduleconfigured to communicate with an arbitrary second NIC via the firstmodule by (i) specifying a network address corresponding to the secondNIC to enable the first module to establish an LTP connection withendpoints at the first NIC and at the second NIC and (ii) exchangingpackets with the second NIC through the LTP connection; and theprocessing hardware further configured to differentiate between thepackets of the network flows and the packets of the LTP connection,wherein packets determined to be packets of the network flows, accordingto their destinations, are handled by being passed to the host and bybeing passed to the network, and wherein packets determined to bepackets of the LTP connection are handled by the first module instead ofbeing passed to the host or the network.
 2. A first NIC according toclaim 1, wherein the second module is further configured to provide datato the first module, and the first module is further configured to causethe first NIC to transmit an LTP packet containing the data as apayload.
 3. A first NIC according to claim 2, wherein the packets of thenetwork flows are received and transmitted by the MAC as payloads ofrespective media frames, wherein the LTP packet is transmitted by theMAC as a payload of a media frame, the packets of the network flows andthe LTP packet comprise Internet Protocol (IP) packets.
 4. A first NICaccording to claim 1, wherein the packets of the network flows compriseTransmission Control Protocol (TCP)/IP packets, wherein the LTP packetis formatted as a User Datagram Protocol (UDP)/IP packet, wherein theTCP/IP packets and the LTP packet comprise an IP address of the hostcomputer connected to the first NIC, and wherein the LTP packetcomprises the IP address of the host computer and an IP address of aremote host computer.
 5. A first NIC according to claim 4, wherein thesecond NIC is connected to the remote host computer, wherein the remotehost computer corresponds to a remote IP address, wherein one of thenetwork flows has endpoints comprising the host IP address and theremote IP address, respectively, wherein the second NIC receives the LTPpacket and, based on an indication of the LTP connection, passes thepayload data to an endpoint module in the second NIC and does not passthe payload data to the remote host, and wherein TCP/IP packets of theone of the network flows are passed through to the remote host by thesecond NIC.
 6. A first NIC according to claim 1, wherein the networkaddress, and a second network address, are both associated with thefirst NIC, wherein the second network address corresponds to the host,wherein the network address corresponds to the LTP connection endpoints,wherein the packets of the LTP connection are addressed to the networkaddress, and wherein the packets of the network flows are addressed tothe second network address.
 7. A first NIC according to claim 6, whereinthe reconfigurable hardware circuitry comprises either a fieldprogrammable gate array (FPGA), an application specific integratedcircuit, an application-specific standard part, or a system on a chip(SoC).
 8. A first NIC according to claim 1, wherein the first module isconfigured to implement the LTP by maintaining sequence numbers forrespective LTP connections, wherein a sequence number is incremented foreach LTP packet sent on a corresponding LTP connection, and wherein thefirst module incorporates a value of a sequence number of the LTPconnection into the LTP packet.
 9. A first NIC according to claim 1,wherein the first module is configured to implement the LTP by (i)buffering LTP packets until corresponding acknowledgment LTP packets arereceived, and (ii) retransmitting buffered LTP packets for whichacknowledgement LTP packets are not received within a timeout period.10. A network interface card (NIC) to handle Internet Protocol (IP)flows, the NIC configured to connect to a host and comprising: a mediaaccess controller (MAC) configured to connect to a network; a controlcircuit configured to pass flow IP packets of the IP flows by: (i)transmitting, from the MAC, flow IP packets received from the host, andby (ii) passing, to the host, flow IP packets received from the networkvia the MAC; and the control circuit further configured to implement alightweight transport protocol (LTP) by establishing and maintainingconnections for the LTP and by providing retransmission of LTP packetsoriginated and sent by the NIC that have been determined by the controlcircuit to require retransmission, the LTP connections having endpointsin the NIC and another NIC, the TLP packets being exchanged between theNIC and the other NIC and not being passed to or from the host.
 11. ANIC according to claim 10, wherein the control circuit is furtherconfigured to implement the LTP by providing ordered delivery of LTPpackets transmitted by the NIC.
 12. A NIC according to claim 10, whereinthe control circuit determines which LTP packets to retransmit based onLTP acknowledgement packets and based on timeouts of transmitted LTPpackets.
 13. A NIC according to claim 10, wherein the control circuit isfurther configured to never pass flow IP packets to an LTP module on theNIC, the LTP module comprising a portion of the control circuit that isconfigured to manage LTP connections.
 14. A NIC according to claim 10,wherein the NIC is configured to receive inbound IP packets, some ofwhich are IP packets of the IP flows, and some of which are IP packetscontaining LTP packets, wherein the control circuit is configured todetermine which inbound IP packets contain LTP packets, and wherein thecontrol circuit is further configured to: by default, for any giveninbound packet received via the MAC that is determined to not contain anLTP packet, based thereon, cause the given inbound IP packet to bepassed to the host, and pass to an LTP module LTP packets from IPpackets determined to contain LTP packets, wherein the LTP module isconfigured to implement the LTP.
 15. A network interface card (NIC) tohandle Internet Protocol (IP) flows, the NIC comprising: a media accesscontroller (MAC), wherein the MAC is configured to receive from anetwork inbound IP packets, wherein some of the inbound IP packets areflow IP packets of IP flows, and wherein some of the inbound IP packetsare IP packets containing lightweight transport protocol (LTP) packetsof an LTP; a control circuit configured to instruct the NIC to pass theinbound flow IP packets of the IP flows to a host; the control circuitcomprising an LTP module configured to implement the LTP by establishingand maintaining connections for the LTP that terminate at the NIC,wherein inbound IP packets that are LTP packets are not passed to thehost; and the control circuit configured to determine which inbound IPpackets contain LTP packets, and: for any given inbound IP packetdetermined to not contain an LTP packet, based thereon, cause the giveninbound IP packet to be passed to the host, and pass, to the LTP module,LTP packets extracted from inbound IP packets that are determined tocontain LTP packets.
 16. A NIC according to claim 15, wherein thecontrol circuit is further configured to retransmit, to the network viathe MAC, LTP packets that have been previously transmitted to thenetwork via the MAC and for which acknowledgments have not beenreceived.
 17. A NIC according to claim 15, the control circuit furtherconfigured to buffer the inbound IP packets, including the IP packetsthat are LTP packets and the IP packets that are not LTP packets,classifying the IP packets, and prioritize transmission of the bufferedIP packets according at least in part to the classifying.
 18. A NICaccording to claim 15, wherein the NIC is configured to connect to thehost via a bus connection.
 19. A NIC according to claim 15, the NICconfigured to maintain a connection table on the NIC, the connectiontable identifying network addresses of remote hosts having LTPconnections that terminate at the NIC.
 20. A NIC according to claim 19,wherein the NIC is configured to maintain the connection table by addingconnection table entries when new LTP connections are opened by the NICand by removing connection table entries when LTP connections are closedby the NIC.